1. Field of the Invention
The present invention relates to a thin display device for use typically in semiconductors, liquid crystal displays and optical parts, and a method for fabricating the display device.
2. Description of the Related Art
An active-matrix liquid crystal display device includes a thin-film transistor (TFT) array substrate, a counter substrate facing the TFT array substrate at a predetermined distance, and a liquid crystal layer sandwiched between the TFT array substrate and the counter substrate. The TFT array substrate includes TFTs as a switching element, pixel electrodes, and a wiring unit containing scanning lines and data lines. The counter substrate includes a common electrode. A passive-matrix liquid crystal display device includes a wiring substrate, a counter substrate facing the wiring substrate at a predetermined distance, and a liquid crystal layer arranged between the wiring substrate and the counter substrate. The wiring substrate contains scanning lines and data lines. The counter substrate contains a common electrode.
As the pixel electrode, an indium tin oxide (ITO) film containing indium oxide (In2O3) and about 10 percent by weight of tin oxide (SnO) is generally used. The data line in the wiring unit to be electrically connected to the pixel electrode comprises, for example, aluminum (Al) or an aluminum alloy such as Al—Nd. A multilayer film of a refractory metal such as molybdenum (Mo), chromium (Cr), titanium (Ti) or tungsten (W) as a barrier metal is interposed between the data line and the pixel electrode, so as to avoid the aluminum or aluminum alloy from being in direct contact with the pixel electrode.
The present inventors have investigated on improvements in a liquid crystal display of this type and have invented an aluminum alloy film that enables direct connection between the wiring unit and the pixel electrode without using such a refractory metal and have filed a patent application as Japanese Patent Application No. 2003-368786.
As possible solutions to keep a low contact resistance between such an aluminum alloy wiring and a pixel electrode without using a refractory metal, Japanese Unexamined Patent Application Publication No. 11-283934 discloses a technique for treating a surface of a drain electrode typically containing an aluminum alloy by plasma treatment or ion injection; and Japanese Unexamined Patent Application Publication No. 11-284195 discloses a technique of forming a multilayer film including a second layer containing impurities such as N, O, Si or C on gate, source and drain electrodes in a first layer, which second layer is to be connected to a pixel electrode.
The aluminum alloy film constituting, for example, the drain electrode to be directly connected to the pixel electrode developed by the present inventors and disclosed in Japanese Patent Application No. 2003-368786 includes aluminum and a metal having a standard electric potential nobler than that of aluminum (−1.7 V), such as Ni (−0.25 V). The standard electric potential refers to an electrode potential with respect to hydrogen ion. Aluminum changes its electrode potential by alloying with the other metal such as Ni. Accordingly, an aluminum alloy film containing aluminum, 2 atomic percent of Ni and 0.6 atomic percent of Nd as disclosed in Japanese Unexamined Patent Application Publication No. 11-284195 has an electrode potential in an alkaline developer at pH of 12.7 of −1.3 V, 0.6 V nobler than that (−1.9 V) of a pure aluminum film in the alkaline developer.
Aluminum is an amphoteric metal, and pure aluminum is inherently not so resistant against an acid and a base. Under regular conditions, however, such an aluminum film has a dense passive film (aluminum oxide layer) on its surface and becomes relatively resistant against corrosion. Corrosion proceeds in various manners. Galvanic corrosion caused by a local cell phenomenon becomes significant in such an alloy. The corrosion rate in galvanic corrosion varies greatly depending on the pH of the developer and the electrode potential of the thin film.
FIG. 1 is a graph showing the relationship between the corrosion of aluminum and the electrode potential and pH of the alkaline developer. Aluminum becomes more susceptible to corrosion with the pH of the alkaline developer approaching 1 or 14 and with a decreasing (becoming more noble) electrode potential. FIG. 1 shows that pure aluminum is less susceptible to corrosion than the Al—Ni—Nd alloy even though it is in the corrosive region.
The corrosion can be prevented if such an aluminum alloy has a nobler potential in a stable region of water (in a region at potentials of −0.75 V to 0.48 V in an alkaline developer of pH of 12.7). It is difficult, however, to allow an alloy mainly containing aluminum to have such a high potential, since the aluminum alloy must keep a low electric resistivity. In other words, an aluminum alloy film in the wiring unit to be directly connected to the pixel electrode becomes more susceptible to corrosion than pure aluminum in terms of designing. It has been experimentally verified that an aluminum alloy has an increased corrosion rate with respect to an alkaline solution.
Among such alkaline solutions (basic solutions) with which aluminum can be in direct contact, alkaline developers for use in developing of a photoresist are strongly basic. Such developers are roughly classified as organic aqueous alkaline developers, inorganic aqueous alkaline developers, and developers in organic solvents. Among them, organic aqueous basic solutions containing tetramethylammonium hydroxide (TMAH) are generally used.
Aluminum does not corrode in developers in organic solvents. It does not significantly corrode in inorganic aqueous alkaline developers, because an inhibitor for inhibiting the corrosion of aluminum can be added thereto. Organic aqueous alkaline developers, however, cannot be incorporated with an inhibitor and may invite corrosion. In addition, a stripping agent containing an amine or its derivative for use in removal of a photoresist becomes alkaline when mixed with water.
If pure aluminum is dipped in such a basic solution for a time set in general fabrication processes, it is significantly prevented from corroding by the anti-corrosion action of the passive film. An aluminum alloy film containing aluminum and an alloy element such as Ni, however, has a nobler electrode potential within the corrosive region as in the pH-potential diagram of FIG. 1. An experiment has verified that an aluminum alloy containing about 2 atomic percent of Ni has an etching rate about five times (about 60 nm/min.) larger than the etching rate (12 nm/min.) of pure aluminum when dipped in an organic aqueous alkaline developer having a pH of 12.7 and containing 2.4 percent by weight of TMAH.
In fabrication of a liquid crystal panel, for example, an alkaline developer comes in direct contact with an aluminum alloy film in a lithography process for forming a wiring pattern on the aluminum alloy film by using a photoresist. The aluminum alloy film is generally etched by using the photoresist as a mask in a subsequent process, and there is no harm in remaining some region not covered by the photoresist or in etching all the region.
Misregistration of patterns formed by using a photoresist, however, may often occur in the photolithography process. In this case, a “rework” process is carried out, in which the photoresist is stripped and the photolithography process is carried out again. If any portion of the aluminum alloy film not covered by the photoresist is corroded in the first photolithography process, the second pattern cannot be registered at the very same position, which may invite a step in the wiring unit as shown in schematic sectional views of FIGS. 2A, 2B and 2C.
More specifically, there is no problem when the aluminum alloy A is etched in exact accordance with a designed pattern by the photoresist P in the first photolithography process (FIG. 2A). However, a pattern failure in which the aluminum alloy A is not covered by the photoresist, if occurred in the patterning process, invites misregistration in the rework process where the pattern is again covered by the photoresist (FIG. 2B), and the resulting step Ax remains to the last (FIG. 2C), which may invite contact failure.